In recent years, attention has been paid to multiband antennas configured such that a single antenna is capable of performing radio wave communication in a plurality of frequency bands. For example, since a radiation electrode performing an antenna operation has a plurality of resonant modes with different resonant frequencies, multiband antennas that are capable of performing radio wave communication in a plurality of frequency bands utilizing a plurality of resonant modes of the radiation electrode have been available.
See Japanese Unexamined Patent Application Publication No. 2004-166242
In general, a multiband antenna utilizing a plurality of resonant modes of a radiation electrode uses a resonance in a fundamental mode with the lowest frequency among the plurality of resonant modes of the radiation electrode and a resonance in a higher-order mode with a frequency higher than that in the fundamental mode. Thus, the radiation electrode is designed such that the resonance in the fundamental mode of the radiation electrode occurs in a lower frequency band among a plurality of frequency bands set for radio wave communication and that the resonance in the higher-order mode of the radiation electrode occurs in a higher frequency band of the settings for radio wave communication.
However, for example, in a miniaturized antenna, due to the constraints of size, it is difficult to separately control the resonant frequency in the fundamental mode of the radiation electrode and the resonant frequency in the higher-order mode of the radiation electrode. Thus, for example, even if the resonant frequency in the fundamental mode can be adjusted to a value that approximately satisfies a request, the resonant frequency in the higher-order mode deviates from an acceptable value. Thus, it has been difficult to form a radiation electrode in which both the resonant frequency in the fundamental mode and the resonant frequency in the higher-order mode can be adjusted to acceptable values.